This three year project is designed to investigate the development of complex sentence forms and meanings in children between 3 and 8 years old. It focuses on the acquisition of sentence complements because they provide a most interesting arena for exploring the interconnections of cognitive and linguistic development. These interconnections have been brought into relief recently by the extensive research activity devoted to the child's developing "theory of mind". The grammatical developments necessary to a mature syntax of complementation are discussed, and subtle tests that can differentiate between children's and adults' knowledge about the attachment of clauses to verbs. These developments will be explored with respect to how they coincide or interact with cognitive developments, such as the child's understanding of false belief or lies, or their ability to draw appropriate inferences with factive verbs. Thus, the research will investigate how syntax, semantics and lexical features of verbs interact in determining the development of complex sentence forms. The methodology will include comprehension or intepretation tasks developed from previous work on children's understanding of wh-questions. However, it will be supplemented by elicited production tasks and naturalistic observations to allow a converging methodology approach. Two year-long longitudinal experimental studies are described, and several cross-sectional experiments with children 3 to 8 years old. The results will inform theoretical work on the development of complex syntax, provide new information on the relation of cognition and language, and aid in the development of more sophisticated diagnostic tests for language delayed children.